1. Field of the Invention
The present invention relates in general to a propulsion device for a watercraft, and in particular to a multiple-jet propulsion device.
2. Description of Related Art
Many watercraft now employ inboard-mounted jet propulsion units due to several distinct advantages over propeller type propulsion systems. For instance, no open propeller poses a hazard with a jet propulsion unit. The unit also does not detract from the watercraft's exterior appearance.
The thrust performance of a jet propulsion unit, however, is commonly limited because the impeller tends to cavitate when driven at a rotational speed above an upper limit. Cavitation reduces the efficiency of the impeller and thus the thrust performance of the jet propulsion unit.
Some prior watercraft have employed several jet propulsion units in order to fully utilize the power output by a high-horsepower engine. The large engine thus drives multiple jet propulsion units, but at a rotational speed that does not cause meaningful cavitation. That is, the engine drives each jet propulsion unit at a rotational speed below the designed upper limit and, thus, cavitation does not occur to such a degree that the efficiency of the jet propulsion unit suffers. The propulsion system thus can provide more thrust without losing efficiency.
Several prior watercraft designs, which employ multiple jet propulsion units, have located the units in a side-by-side arrangement and behind a common, centrally disposed water inlet located on the underside of the watercraft hull. The use of a common, centrally located water inlet reduces the tendency of air being drawn into the jet propulsion units when tuning, as well as provides more continuity of the hull undersurface.
A common water inlet used to feed two or more jet propulsion units, however, often compromises the performance of the jet propulsion units. Each impeller of the jet propulsion unit tends to produce a vortex in the water flow on the inlet side of the unit. This swirling motion in the water flow upstream of the units can cause a counter-effect between the side-by-side units that tends to decrease the efficiency of the jet propulsion units.